Shapewear after fitting?

I would cautiously say yes. I only had one fitting, but she didn't any hemming because I didn't have my shift. But she did fit the top of my dress against my body, following the lines of the shapewear. I think it depends on your dress - is it a zipper, buttons, corset? With a corset or straps it probably wouldn't change as much. I found a lot of the support was in the boning of the chest, so you might want to hold off alterations until you find shapewear. Or have her do the hemline with your shoes the first time and worry about the top with your second fitting.

The only way I can see this being a problem is if it takes more than an inch or two off the measurements your current shape-wear provides. If you went from say, spanx to a steel-boned corset that would be a problem, but most spandex/lycra shape-wear doesn't provide that much difference in inches

My shapewear requires adding lace in the back because it's rounded vs straight across. Try Spanx THINSTINCS. Specifically that kind. It pulls you in, and is much thinner and a smooth non-clingy fabric. Light and smooth enough for under my lightweight lace dress

Have you tried Shapermint? Most comfortable shapewear I've ever had.